I'm designing a RS485 bus with the MAX3485 chip (half duplex) and I'm having a problem in sending message packets to other nodes in the bus.
After some debugging I came across this:
A MAX3485 powered by 3.3V that is not connected to the bus and has /RE=0 (read is enabled) has 0V in the RO pin, which I think is the right value since it's not receiving any data. The bus lines D+ and D- have a 100 ohm load resistor. If I remove this resistor, RO changes to +3.3V.
On the other hand, another different board but with the same microcontroller and the same MAX3485, in the same conditions: no load and /RE=0, has RO at +3.3V, no matter if there is the 100 ohm resistor or not.
I guess this is the reason why I'm getting problems in the communication.
Do you have any clue about what's happening?